1. Field of the Invention
The invention relates to a method for preserving agricultural products which are used for animal feed and a Lactobacillus plantarum inoculant for use in the method.
2. Brief Description of the Background Art
Spoilage of stored agricultural products is a major problem in the farming industry. This spoilage often arises during long-term storage of these agricultural materials. Typically, long-term storage is necessary to provide adequate supplies of feed for domestic animals during those times of the year when fresh feeds are unavailable.
The storage of silage, baled hay, and storage of high moisture grain are among the most common approaches for providing an adequate supply of feed for those months when fresh feeds are unavailable.
Silage is produced by the anaerobic fermentation of the materials described above. Typically, the material is vegetable matter which is cut and then loaded into a silo, most commonly a vertical cylindrical structure, and kept anaerobic to inhibit the growth of aerobic spoilage organisms. Ideally, under anaerobic conditions the homofermentative bacteria proliferate, producing lactic acid which, by lowering the pH of the environment, further inhibits the growth of undesirable organisms. Unfortunately, the contents of the silo often become exposed to air due to leaks in the silo structure or during the gradual removal of silage. Such re-exposure to aerobic conditions allows proliferation of fungi to occur and results in spoilage of the silage.
In most circumstances, hay is cut and allowed to dry in the field until the moisture level has dropped to 8-15%. This drying process is performed because hay baled at higher levels of moisture is much more likely to degenerate due to the influence of heat of decomposition and the activity of epiphytic spoilage organisms. Unfortunately, while drying the hay to 8-15% moisture substantially avoids these detrimental effects, these low levels of moisture result in high field losses due to drying and hay of diminished nutritional value due to loss of the leafy part of the plant. It is, therefore, most desirable to bail hay at moisture levels of 25-30% if deterioration due to heat and microorganisms can be controlled.
The high moisture grain is stored in silos under anaerobic conditions, as described above for silage. Spoilage of high-moisture corn is due primarily to fungi which proliferate under storage conditions. Candida guilliermondii, Candida pelliculosa, Hansemula anomala, Penicillium spp., and Aspergillus spp. are among those fungi often responsible for the spoilage of high-moisture grains and silage.
Often the materials which are preserved using these processes contain naturally-occurring opportunistic epiphytes. These epiphytes may be beneficial or detrimental in determining the eventual status of the preserved agricultural material and compete among themselves for ecological dominance in the stored material.
The predominance of undesirable organisms can result in spoilage and contamination of the feed material such that it can no longer be used to maintain domestic animals. These undesirable organisms may exert their effect by destroying the nutritive value of the feed, or even by producing toxins which are dangerous to the well-being of the animals. A common source of spoilage organisms is found in storage facilities and on associated processing equipment. These spoilage organisms may be either bacteria or fungi. Common bacterial spoilage organisms are those which are members of the genera Clostridium and Listeria. Fungal spoilage is often caused by members of the genera Aspergillus, Candida, Mucor, and Saccharomyces.
Various approaches have been made to limit the proliferation of those epiphytes and storage organisms responsible for spoilage of agricultural products. Since the desirable epiphytes are tolerant of low pH, while spoilage organisms such as Clostridium spp. are not, one simple approach has been to spray the agricultural material at the time of storage with an acid as, for example, propionic acid. A major disadvantage of this approach is its cost since large quantities of acid must be used to adequately treat the agricultural material. In addition, the inherently toxic and corrosive nature of propionic acid requires that special handling techniques be used by the farmer to protect personnel handling the acid. Further, all equipment coming in contact with the acid must be thoroughly decontaminated to avoid corrosive damage.
Another approach to limiting the growth of undesirable organisms during the storage of agricultural materials has been to inoculate these materials with those epiphytes thought to be beneficial to the preservation of the agricultural material such that the epiphytes will be able to out-compete and thereby, limit the proliferation of the spoilage organisms. Those organisms which were considered most beneficial in the past have been selected primarily on the basis that they were homofermentative lactic acid producers. The rationale for this approach is that the low pH caused by the lactic acid produced by these organisms would inhibit the growth of spoilage organisms. Strains from species of the genera Lactobacillus, Streptococcus, and Pedicoccus are among those considered to be most beneficial in achieving this type of inhibitory effect.
Lactic acid bacteria have often been used in the preservation of food products utilizing fermentation processes. For example, Jeffreys, U.S. Pat. 3,677,897, discloses a method of culturing and preserving lactic acid bacteria for use as starter cultures in various fermentation processes. The patentee states that the method of the invention can be used to stabilize any of the lactic acid bacteria, singly or in combination, which are well known in the dairy and fermentation industries. Among those organisms which are mentioned is Lactobacillus plantarum. The method comprises coating the organisms with an acetylated monoglyceride and adding a carrier selected from the group consisting of modified cellulose and modified starches as, for example, bran solids.
In Moon (Journal of Science, Food & Agriculture, 32: 675 (1981)), the author studied the effect of experimental silage prepared from green bean and potato processing wastes which were inoculated with a strain of Lactobacillus plantarum. Since these materials were very high in moisture, it was necessary to add ground peanut hulls to reduce the percentage of moisture prior to inoculation. In producing silage in this manner, the author reports that large numbers of undesirable organisms were only present in the uninoculated control silages and that overall the addition of L. plantarum had a beneficial effect on the fermentation of this silage material.
Woolford et al. (Grass & Forage Science, 39: 139 (1984)) describes studies on the effect of different cultures of lactic acid bacteria on silage fermentation. The authors set up experimental silages and evaluated 21 strains of lactic acid bacteria from various genera and species, in terms of meeting certain criteria which the authors believe are indicative of the ability to produce good silage. The authors state that none of these cultures satisfied all of the criteria, but three strains (Streptococcus durans, strain 1024; Lactobacillus acidophilus, strain 2356; and Lactobacillus plantarum, strain 6) had greater potential for producing good farm silage than the other 18 strains tested.
In a study reported by McMahon et al. (Applied Microbiology, 30: 103 (1975)), the authors studied the deterioration of high-moisture corn in leaky silos which were filled with either normal high-moisture corn (HMC) or with HMC severely infested with Southern corn leaf blight fungus (Helminthosporium maydis). The authors determined the number of mesophilic bacteria, lactobacilli, coliforms, yeast and molds present on the corn samples as received and periodically during 220 days of storage. The authors state that Lactobacillus plantarum was the most frequent bacterial isolate, although it was not as predominant in the normal corn as had been found in a previous study. Indeed, L. plantarum was found at greater frequency in the blight infested corn than in the normal corn.
In Dellagio et al. (Systemic & Applied Microbiology 5: 534 (1984)), the authors studied the lactic acid bacteria present in high-moisture corn grain for up to 120 days using DNA-ONA hybridization. The authors determined that all of the homofermentative lactobacilli tested were highly related to the type strain of Lactobacillus plantarum.
The American Type Culture Collection has publicly available a strain of Lactobacillus plantarum (ATCC 14431) which was isolated from grass silage.
However, in spite of extensive research into the isolation and development of various organisms for use as agricultural product inoculants, spoilage of these products has remained an on-going problem.